Tsunamis are unlike waves formed by tides or surface winds. A tsunami radiates out in all directions from the epicenter of an undersea earthquake. It may hit nearby coastal areas in seconds, or it may travel thousands of miles across open seas before striking land.
On the open sea, away from shallow areas and coastlines, a tsunami is virtually undetectable, with waves that crest at less than 3 feet, and crests that may be separated by hundreds of miles. In fact, ships at sea are generally unaware that they are riding a tsunami, which is why there have been no reports of damage or distress from any cruise ship, freighter or other vessel at sea in the region. However, hundreds of pleasure boats and fishing boats close to shore or docked in marinas in areas hit by the tsunami have been destroyed.
A tsunami's speed is determined by the depth of the ocean, and this one moved at speeds in excess of 500 miles per hour as it raced toward Sri Lanka, India and Africa. Tsunamis cannot be seen from the air or sea until they enter shallow water. It is there that the tsunami slows dramatically, to 20 or 30 miles per hour, and begins to rise above sea level. This tsunami was most likely imperceptible in deep water, but it rose to a height of 20 to 40 feet before it slammed into shore.
Tsunamis drain the water away from the beach before they strike, and in some places, the water retreated hundreds of yards. There are eyewitness accounts from snorkelers who suddenly found themselves face down in wet sand or atop sharp coral.
Then the water began to rise in the distance, and a sound that has been described as being similar to a jet engine was heard. Beachgoers had at most a few minutes to run for higher ground by the time they heard this sound, but of course, few knew what to make of the signs until it was too late.
Tsunamis don't break like normal waves, they simply rise and careen forward. Wind-generated waves are typically 5 to 20 seconds apart, while tsunami waves may be separated by minutes or even hours. A tsunami will typically consist of 3 to 10 waves, and each wave can carry objects or people out to sea as it retreats.
More than 95% of the world's earthquakes occur beneath the Pacific Ocean, and early warning systems are in place to detect tsunamis that might threaten the Pacific Coast of the U.S., Alaska, Hawaii, and many Pacific Rim countries. No such system exists in the Indian Ocean, though governments there will surely go to the expense to deploy one now.
# posted by masquerader @ 15:49 ::
